1. Technical Field
The present invention relates to apparatuses and methods for accelerated activation of a fuel cell, which can stably and rapidly secure a high stack performance while significantly improving performance of a polymer electrolyte membrane fuel cell, minimize hydrogen usage for the activation, and simplify the structure of the apparatus.
2. Background Art
A polymer electrolyte membrane fuel cell has excellent efficiency, high current density, high output density, short start-up time, and a fast response characteristic to a load variation, in comparison with other types of fuel cells,
Particularly, a polymer electrolyte membrane fuel cell is less sensitive to a pressures variation of reactant gas and has the output of various ranges. For these merits, a polymer electrolyte membrane fuel cell has been employed in various industrial fields as power sources of, for example, a zero emission vehicle, a private power station, a military purpose, and so on.
A polymer electrolyte membrane fuel cell generates water and heat through an electrochemical reaction between hydrogen and oxygen. Supplied hydrogen is decomposed into a hydrogen ion and an electron by a catalyst in an anode. The decomposed hydrogen ion is transferred to a cathode through an electrolyte membrane.
At this time, oxygen supplied to a cathode is combined with the electron transferred to a cathode through an external conductive wire to thereby generate electronic energy while generating water. In this case, an ideal electrical potential is about 1.3V, and it can be expressed as a following chemical reaction equation:Anode:H2→2H++2e−Cathode:½O2+2H++2e−→H2O
Heat generated in a unit cell due to the electrochemical reaction can be expressed by the following formula:Q=I×(1.23−V)
where Q denotes a generated calorific value, I denotes an electric current capacity, and V denotes an average of generated voltages.
To obtain an electrical potential necessary for a vehicle, individual unit cells have to be stacked together until a desired electrical potential is achieved. The unit of stacked cells is referred to as a stack (i.e., fuel cell stack). FIG. 1 shows a configuration of a fuel cell stack.
The fuel cell stack of FIG. 1 is manufactured such that unit cells are assembled with a current collecting plate 104, an insulating plate 105, and a coupling plate 106 having a high voltage terminal 107 by using a coupling band 108 and a bolt 109. Here, the unit cell comprises a membrane electrode assembly (MEA) 100 which includes an electrolyte membrane for transferring hydrogen protons and catalyst layers (cathode and anode) coated on both sides of the electrolyte membrane so that hydrogen and oxygen can react, a gas diffusion layer (GDL) 102, and a separator 103 for supplying fuel and draining water generated by an electrochemical reaction between hydrogen and oxygen.
In the fuel cell stack essentially having the above-described configuration, hydrogen is supplied to an anode, and air or oxygen is supplied to a cathode, so that an electrochemical reaction occurs therein, generating electrical energy of high efficiency and water. That is, an electrochemical reaction happens due to the reactant gases in the catalyst layer of the fuel cell, and hydrogen ions generated move through an electrolyte in the catalyst layer and the electrolyte membrane, and electrons move into an electricity generating device through the catalyst, the gas diffusion layer, and the separator.
Particularly, hydrogen ions which move through the electrolyte or the electrolyte membrane moves through water which exists in the electrolyte membrane, and so the electrolyte in the catalyst layer and the electrolyte membrane must be sufficiently hydrated in order for the fuel cell to have better performance, and the reactant gases must smoothly arrive at the catalyst layer for the electrochemical reaction.
In such a fuel cell stack, electrodes such as a cathode and an anode are catalyst layers made by combining a hydrogen ion carrier such as nafion with a catalyst such as platinum. Such a fuel cell stack may become less activated due to an electrochemical reaction which occurs at an initial driving stage of a manufactured fuel cell. The reasons are as follows.
Firstly, a reactant cannot reach a catalyst since a reactant passage is blocked.
Secondly, a hydrogen ion carrier such as nafion, which forms a triple phase interface with a catalyst, is not easily hydrated at an initial driving stage.
Lastly, continuous mobility of hydrogen ions and electrons is not ensured.
For these reasons, an activation process is required so as to ensure a performance of a fuel cell to the maximum extent after a fuel cell is assembled.
The purpose of an activation process, which is also known as a pre-conditioning process or a break-in process, is to activate a non-reactive catalyst and to sufficiently hydrate an electrolyte contained in an electrolyte membrane and an electrode, thereby ensuring a hydrogen ion passage.
An activation process is carried out to make a fuel cell reach its maximum performance after a fuel cell is assembled. This may take a number of hours or days based on a driving condition. For example, due to an improper activation process, a fuel cell may be driven in a state that it does not reach at its maximum performance.
As described above, an improper activation process reduces a throughput of a fuel cell and increases hydrogen usage, thereby increasing a stacking cost and degrading a stack performance.
Fuel cell manufacturers have proposed methods of activating a fuel cell. A typical activation method is to drive a fuel cell for a long time under a specific voltage.
For example, an activation method of a solid polymer fuel cell is disclosed in Japanese Patent Application No. 2003-143126, assigned to AISIN SEIKI Co. Ltd. According to Japanese Patent application No. 2003-143126, a fuel cell stack is exposed to a low voltage for a long time, so that even a portion where its stack performance is no longer improved is activated. In this instance, however, there is a disadvantage in that it takes a long time for a fuel cell to reach its maximum performance even though an activation process is simple.
As another conventional method, a method of activating a polymer electrolyte membrane fuel cell using a step-voltage driving is disclosed in Korean Patent Application No. 2005-0120743, assigned to an assignee of the present invention. According to Korean Patent Application No. 2005-0120743, a voltage cycle of from an open circuit voltage (OCV) (1 minute) to 0.4 volt (5 minutes) is applied to a stack at a high driving temperature (70° C.) and at relative humidity (RH) of 100% (see FIG. 3), thereby reducing an activation time of a fuel cell to within about 3 hours. In this case, however, there is a problem in that if a time to obtain a maximum output after a stack is manufactured is lengthy, time efficiency can be lowered, which will reduce and price competitiveness can be lowered since hydrogen and oxygen are excessively consumed while driving.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.